This invention relates to a method of inspecting a resist layer on a gravure printing plate, and a device for practicing the method.
In general, the material of a gravure printing plate is copper. Recesses different in depth according to the light and shade of a picture to be printed are formed in a copper plate to provide a printing plate. The recesses are formed by etching the copper plate with an etching liquid essentially containing ferric chloride. In this etching operation, a gelatin resist layer (carbon tissue) is used as resist in etching. The gelatin resist layer is partially different in thickness according to the light and shade of a picture to be printed before the gravure printing plate is subjected to the etching, so that the etching liquid reaches the copper plate in different times depending on the different thickness. That is, as the total time required for etching the copper plate is controlled by the presence of the gelatin resist layer, the recesses different in depth according to the light and shade of the picture are formed in the copper plate, whereby the aimed printing plate is provided.
The time required for the etching liquid to permeate the gelatin resist layer depends not only on the thickness of the gelatin resist layer but also other various factors such as the density of the etching liquid, the temperature of the etching liquid, and the amount of water contained in the gelatin. Accordingly, the processes of manufacturing gravure printing plates are strictly controlled. However, the characteristics of a gelatin resist layer are liable to be varied by unexpected factors. Therefore, the fluctuations characteristic of the gelatin resist layer are, in general, corrected by adjusting the etching conditions while etching is being carried out. This method may be effective in the case where the degree of fluctuation in characteristic of the gelatin resist layer is small. However, if the degree of fluctuation is large, then it is impossible for the method to adjust the etching conditions suitably, as a result of which the depths of the recesses formed by etching the copper plate become out of the allowable depth range, and the resultant printing plate cannot be used. In addition, the method is disadvantageous in that it takes a relatively long period of time to manufacture a gravure printing plate in addition to the waste of the material (or the printing cylinder).
In order to overcome this difficulty, a visual inspecting method has been employed, in which the degree of advancement of etching is visually inspected with a tone scale or by referring to a picture to be formed.
When the etching liquid essentially consisting of ferric chloride (FeCl.sub.3) permeates the gelatin resist layer to reach the surface of the copper plate of the printing plate, the copper (Cu) reacts with the etching liquid to produce cupric chloride (CuCl.sub.2), and the ferric chloride (FeCl.sub.3) becomes ferrous chloride (FeCl.sub.2). ##EQU1##
In general, the gelatin resist contains a red pigment, and therefore its color is similar to that of the surface of the copper plate. However, as the copper plate is etched, the color of the gelatin resist layer looks black. Therefore, by observing this change, permeation of the etching liquid can be confirmed. On the other hand, the characteristic of the gelatin resist layer can be inspected by comparing the time interval which elapses from the time instant that the etching liquid is dropped until etching is started with a reference time. This inspection method can be achieved readily by dropping the etching liquid onto a region of the gelatin resist layer, which does not directly relate to printing. However, the inspection method is still disadvantageous in the following points:
(1) The time required for the inspection is substantially equal to that required for actually carrying out the etching.
(2) If an etching liquid low in density and high in permeation speed is employed as the inspecting etching liquid, and mutual relation between the inspecting etching liquid and the etching operation is suitably established, then the inspection time may be reduced. However, even if the etching liquid reaches the surface of the copper plate, the gelatin resist layer does not change its color abruptly. In addition to this, the arrival of the etching liquid to the surface of the copper plate is detected visually. Thus, errors in detection are similarly caused.
(3) In this inspection, the inspection region is etched. Therefore, the inspection region must be selected at a position on the printing plate, which does not directly concern printing; however, it is often difficult to select such a position.
(4) If, as a result of the inspection, the resist layer is found so unsatisfatory that it cannot be corrected in the following process, then the resist layer should be removed from the printing plate so that the latter can be used again. However, the inspection region, being etched, will become an obstruction against the re-use of the printing plate.
As the inspection carried out before the etching process has no particular merits as described above, in the existing circumferences the above-described etching operation is carried out while the characteristic of the resist layer is being inspected. In other words, while the degree of advancement of etching is being visually determined with a tone scale or by referring to a picture to be printed, etching liquids different in permeation speed are selectively used according to whether the etching speed is higher than or lower than a reference value, so that the etching operation is achieved within the predetermined allowance. However, in such a method, the characteristic of the resist layer is inspected indirectly in the course of etching control. Therefore, the method is disadvantageous in that, even if any etching control method is employed, the etching is advanced over its control range, as a result of which the printing plate is finished unsatisfactorily.